Multi-roll calenders are used in the finishing of the paper web, either in connection with the paper machine (ON-line) or separately from the paper machine (OFF-line). A multi-roll calender is formed of several rolls set on top of each other, the surfaces of which are alternately hard and soft.
Particularly precisely in ON-line calenders, the tail-threading procedure takes place at the running speed of the paper machine, which nowadays is already nearly 2000 meters a minute. This makes demands on the success of the tail threading, which have been attempted to be resolved in several different ways.
Solutions are known, in which a tail threading rope is used to guide the threading tail through the set of rolls. Besides rope threading, various kinds of air blows can be used as an aid. As is known, rope threading is, however, a way of threading the tail that is uncertain, has a limited threading path, and is even questionable in terms of work safety, especially at high speeds, which has led to attempts to develop more advance solutions in stead of it.
U.S. Pat. No. 6,245,198 discloses suction boxes to be used along with a tail-threading belt, which are set in the free spaces between the calender rolls and the web take-off rolls. The suction boxes are used to support the web against the tail-threading belt.
Another example of the prior art is represented by the pulling nip arrangement disclosed in utility model FI-4362. In this, the web threading tail is taken, in the first sequence, using rope threading, through the calender nips, to a point preceding the lowest nip, in which a so-called pulling nip is arranged in connection with the web take-off roll. After the pulling nip, the paper web is led down to broke processing. When moving to the second sequence, an intermediate cut is made after the pulling nip, after which the threading tail is guided through the last roll nip and on, for example, to a reeler. In this arrangement, however, the suction belt preceding the cutting point, i.e. the holding point of the web threading tail, is disadvantageously far away. This causes slack after the holding point at the moment of cutting which can break. In addition, the functionality of such an arrangement sets an upper speed limit of less than 2000 meters a minute.
FI publication 20001965 discloses a solution for closing the tail threading of a multi-roll calender. In it, the web is supported between the roll of the calender and the take-off roll by at least one means of a shut-off element not extending through nip. The shut-off element permits tail threading without separate devices in the roll stack. The solution also permits tail threading that proceeds in stages and covers the full web width. Using such ropeless supported threading, speeds of considerably more than 2000 meters a minute can already been achieved.
Further, tail-threading methods arranged using suction belts are also known, and can be exploited in, for example, the solution disclosed in the utility model FI-4362. In practice, however, their functionality is limited by complicated threading geometries, which cause the tail to peel off the belt as it proceeds. A peeled-off double tail is then formed, which significantly hampers the tail threading, and finally prevents it altogether.
Further, despite even the more developed tail threading solutions described above, the ropeless tail threading process is typically hampered in multi-roll calenders by the increase in the threading distance, changes in the direction of travel of the threading tail (to a greater extent the larger the change in direction), and the threading through open roll nips.